Among fuel cell systems which generate power by reacting a fuel with an oxidizer, particularly polymer electrolyte fuel cells are used in power supplies for automobiles that run by means of electric motors, and household cogeneration systems because they are capable of generating power at a relatively low temperature of about 100° C. and have a high power density.
Usually, the polymer electrolyte fuel cell includes a fuel gas containing hydrogen and an oxidizer gas containing oxygen, the fuel gas being isolated from the oxidizer gas by an electrolyte membrane. The side to which the fuel gas is fed is referred to as an anode side, and the side to which the oxidizer gas is fed is referred to as a cathode side. The fuel gas fed to a groove of a separator on the anode side diffuses into a gas diffusion electrode that is in contact with the separator, and the fuel gas is separated into electrons and protons at an anode catalyst layer arranged on the other surface (surface opposite to the side that is in contact with the separator) of the gas diffusion electrode. Electrons are connected to a load (device) outside the fuel cell through carbon particles in the catalyst layer and carbon fibers which form the gas diffusion electrode, so that a direct current can be extracted. The electrons move to the cathode catalyst layer through the gas diffusion electrode as a cathode, and protons generated at the anode catalyst layer move to the cathode catalyst layer through the electrolyte membrane. The oxidizer gas containing oxygen is fed to a groove of a separator on the cathode side, and diffuses into the gas diffusion electrode that is in contact with the separator, and the oxidizer gas generates water together with protons and electrons at a cathode catalyst layer arranged on the other surface of the gas diffusion electrode. The generated water moves to the groove of the separator on the cathode side through the gas diffusion electrode from the catalyst layer, and passes through the groove of the separator to be drained outside the fuel cell.
In the polymer electrolyte fuel cell, when the gas diffusion electrode is densified for obtaining electrical conductivity and thermal conductivity, diffusion of hydrogen and oxygen necessary for a reaction may become insufficient. In addition, it may be unable to obtain high power generation efficiency due to occurrence of so called flooding in which water generated in the reaction fills voids in the catalyst layer and the gas diffusion electrode to prevent transportation of hydrogen and oxygen. On the other hand, when ionomers in the electrolyte membrane and the catalyst layer are not sufficiently humidified, and thus drying is accelerated to cause so called drying-out, it may be unable to obtain high power generation efficiency. In view of these problems, an attempt has been made to improve drainage of water by, for example, a method in which a gas diffusion carbon fiber nonwoven fabric is subjected to a hydrophobic treatment with a fluororesin etc., and a method in which a micropore layer (hereinafter, referred to as a microporous layer) formed of a fluororesin and electrically conductive particles is formed on a gas diffusion electrode, but the effect thereof is not sufficient, and further improvement is desired.
For example, Patent Documents 1 and 2 disclose a technique in which a carbon paper provided with pores each having an opening on the channel side is used as a gas diffusion electrode to smoothly drain generated water through the pores.
Patent Documents 2 and 3 disclose a technique in which non-through pores having a depth equivalent to 20 to 80% of the thickness of a gas diffusion carbon fiber nonwoven fabric are formed in the gas diffusion carbon fiber nonwoven fabric by laser processing to secure both drainage of generated water and moisture retainability of ionomers in an electrolyte membrane and a catalyst layer.